Aqueous mixtures of two polymers such as poly(ethylene glycol) (PEG) and dextran can separate spontaneously into two aqueous phases, called aqueous two-phase systems. Phase separation in aqueous solutions of polymers is an extraordinary and underexplored phenomenon. When two aqueous solutions of polymers are mixed, the resulting system is not homogeneous; rather, two discrete phases, or layers, form. These layers are ordered according to density and arise from the limited interaction of the polymers for one another. In these systems, each phase predominantly consists of water (upwards of 70-90% (w/v)), while the polymer component is present in concentrations ranging from micromolar to millimolar. A low interfacial tension and rapid mass transfer of water-soluble molecules across the boundary characterize the interface between layers.